A nitrogen-oxide gas includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O) and is represented by NOx. Nitric oxide (NO) and nitrogen dioxide (NO2) are mainly included in the nitrogen-oxide gas and act as atmospheric pollution sources so that a discharge amount thereof may need to be appropriately controlled by measuring the concentration.
A general method of measuring concentration of nitrogen-oxide includes use of equilibrium potential. In the use of the equilibrium potential, solid-state nitrate is formed as a sensing electrode in a solid electrolyte and a noble metal electrode is formed to make ion activity uniform in the solid electrolyte so as to form an electrochemical cell. Thus, an electromotive force generated from the cell is used to measure concentration of nitrogen-oxide. However, such a method is hardly applied to a gas in high-temperature due to low melting point of a sensing electrode.
Another method of measuring concentration of nitrogen-oxide includes use of a current type sensor. In the use of the current type sensor, an oxygen pumping cell is used to convert nitrogen dioxide into nitric oxide, and a current by oxygen ion obtained by decomposing nitric oxide is measured, thereby measuring concentration of nitrogen-oxide. However, in this method, there is structural limitation such as use of an oxygen pumping cell, current measurement by oxygen ion is considerably changed according to temperature, and a measured current is significantly decreased under few hundreds ppm or below so that a total amount of nitrogen-oxide is hardly measured.
Another method of measuring concentration of nitrogen-oxide includes mixed dislocation. In the mixed dislocation, a sensing electrode is formed at one side of an oxygen ion conductive solid electrolyte by using metal oxide, and a reference electrode is formed at the other side of the solid electrolyte by using a noble metal so that a potential difference between the sensing electrode and the reference electrode is measured. That is, the sensing electrode has reactivity against nitrogen-oxide and oxide, whereas the reference electrode only has reactivity against oxygen so that a potential difference between the sensing electrode and the reference electrode is generated according to concentration of nitrogen-oxide included in a gas and thus the potential difference is measured so as to measure concentration of nitrogen-oxide. However, in this method, due to a difference of an electromotive force generated according to decomposition of nitrogen dioxide and nitric oxide, measurement accuracy of a nitrogen-oxide gas including both nitrogen dioxide and nitric oxide is decreased.
In this regard, a method of using a conversion cell that convert nitrogen-oxide into one gas type has been suggested. However, there is a limit to covert entire nitrogen-oxide into nitric oxide or nitrogen dioxide and thus it is hard to measure concentration of nitrogen-oxide.